While there are needs for a sealant for solid oxide fuel cells (SOFC), it is prerequisite that such a sealant would neither degrade nor melt even after exposure to high temperatures for a long time, for it comes to be exposed to such high temperatures as 600-800° C. for an extended length of time. Crystallized glass hase thus been proposed. Also required is flowability of the glass during firing in order for providing a seal between metal and ceramics.
On the other hand, many of the crystallized glass compositions for sealing SOFC that have been developed so far include B2O3 and alkali metal oxides (Patent Documents 1 and 2). However, there has emerged a problem of contamination of electrodes with evaporating glass components such as B2O3 while such a sealant is kept at high temperatures, and therefore, it is desired to develop a glass composition free of evaporating components, in particular, B2O3.
Though some glass compositions free of B2O3 have been developed (Patent Documents 3 and 4), they have a drawback that they have insufficient flowability because of their small ZnO content. While there is an example of glass composition that has been developed free of B2O3 and alkali metal oxides (Patent Document 5), that glass compound contains not more than 43 mol % alkaline earth oxides RO (R: Mg, Ca, Sr, Ba, Zn) as glass-modifying oxides, and consequently, includes a large content of a glass-forming oxide (SiO2), which results in an insufficient crystallization during firing, and further has drawback of poor flowability because of its small ZnO content. Further, while a SiO2—MgO-based glass composition has been disclosed (Patent Document 6), the composition, which is free of B2O3, has also a drawback of poor flowability coming from its low ZnO content.